Ion detectors including a combination of a conversion dynode and a secondary-electron multiplier are often used for detecting ions with high sensitivity in a mass spectrometer. In such an ion detector, a high voltage (± several [kV] to ±10 [kV] for example) having a polarity opposite to that of the ions to be analyzed is applied to a conversion dynode for selectively detecting positive ions and negative ions. In a liquid chromatograph mass spectrometer, an ion source according to an electrospray ionization (ESI) method, for example, is used for vaporizing and ionizing a liquid sample. In such an ion source, a high voltage (± several [kV] for example) with the same polarity as that of the ions to be analyzed is applied to the tip of a nozzle for spraying the liquid sample.
In these applications, the polarity of the high voltage to be applied needs to be changed in response to the polarity of the ions to be analyzed. Therefore, a high-voltage power unit having an output voltage capable of switching the polarity of the output voltage is used. One of the most conventionally well-known high-voltage power units for switching high voltages of different polarities is one using a high-voltage reed relay (see, for example, Patent Literature 1).
In a high-voltage power unit using a reed relay, when switching the polarity of the output voltage, spike discharges may occur to break the relay. In order to avoid such a situation, it is necessary to observe the following procedures: decrease the output voltage in one polarity, actuate the relay to change the contacts when the output voltage becomes adequately low, and after that, increase the output voltage in the other polarity. Consequently, it takes some time to switch the polarity. In the case where, for example, the detection of positive ions and negative ions are alternately switched every short period of time, the non-detection time increases in a mass spectrometer. This causes a problem of affecting the accuracy of an analysis.
As a solution to such a problem, a high-voltage power unit capable of switching the polarity of an output voltage at high speed is disclosed in Patent Literature 2. FIG. 7 is a circuit configuration view of principal parts of the high-voltage power unit, and waveform charts (a), (b) and (c) in FIG. 8 are waveform charts illustrating change in a voltage in the case of switching the polarity in the high-voltage power unit. With reference to FIG. 7, and the waveform charts (a), (b) and (c) in FIG. 8, a configuration and operation of the high-voltage power unit will schematically be described.
In the high-voltage power unit, a positive voltage generating circuit 2 includes a transformer T1 as a booster, a drive circuit 3 for driving a primary winding of the transformer T1, and a rectifier circuit using a Cockcroft-Walton circuit composed of four capacitors C1 through C4 and four diodes D1 through D4 connected to the secondary winding of the transformer T1. A negative voltage generating circuit 4 is similar in a basic configuration to the positive voltage generating circuit 2 except for the point that the direction of each of the diodes D5 through D8 in the Cockcroft-Walton circuit is opposite to that in the positive voltage generator 2.
An output terminal P2 of the positive voltage generating circuit 2 and an output terminal Q1 of the negative voltage generating circuit 4 are connected. The other output terminal Q2 of the negative voltage generating circuit 4 is grounded via a resistor 9. Between the output terminals P1 and P2 of the positive voltage generating circuit 2, a resistor 51 is connected in parallel. Between the output terminals Q1 and Q2 of the negative voltage generating circuit 4, another resistor 52 is connected in parallel. A high voltage is output from the output terminal P1 of the positive voltage generating circuit 2 where the polarity is switched. Between this high-voltage output terminal and the ground, a resistor 7 and a resistor 8 are connected in series. A voltage signal is fed back to a control circuit 1 from a junction point between the resistors 7 and 8.
The drive circuits 3 and 5 each include a direct current voltage supply, which is connected in series to the primary winding of the transformer T1, and a switching element. The voltage applied from the direct current voltage supply to the primary winding (or a current to be supplied) is connected and disconnected by the switching element. The pulse width of a rectangular wave signal that performs ON/OFF driving of the switching element is controlled by the control circuit 1. Accordingly, the effective electric power supplied to the primary winding of the transformer T1 is changed, and consequently output voltages of the positive voltage generating circuit 2 and the negative voltage generating circuit 4 are changed.
To output a positive high voltage +HV, the control circuit 1 sends a driving control signal only to the drive circuit 3 on the positive voltage generating circuit 2, so that only the positive voltage generating circuit 2 is operated and the negative voltage generating circuit 4 is stopped. In this case, since a voltage value corresponding to the voltage +HV appearing at the high-voltage output terminal is supplied to the control circuit 1 as a feedback, the control circuit 1 compares this voltage value with a target value, and regulates the driving control signal supplied to the drive circuit 3 in order to reduce the error. Accordingly, the output voltage +HV is precisely set to any target voltage. Contrary to the above case, to output a negative high voltage, the control circuit 1 sends a driving control signal only to the drive circuit 5, so that only the negative voltage generating circuit 4 is operated while the positive voltage generating circuit 2 is stopped.
During a transition period wherein output of the positive high +HV is switched to output of a negative high voltage, the control circuit 1 controls each of the drive circuits 3 and 5 so that the output of the positive voltage generating circuit 2 changes from the voltage +HV to zero, while simultaneously the output of the negative voltage generating circuit 4 changes from zero and subside on a voltage −HV after an overshoot (see waveform charts (a) and (b) in FIG. 8). Thus, by deliberately overshooting the voltage that changes from zero in this way, delay in a fall of the other voltage that returns to zero is compensated for. This makes it possible to quickly reach the target output voltage. Accordingly, the output voltage changes in a short period of time.
In recent years, to meet demands for further enhancement in throughput and higher temporal resolution in measurement in mass spectrometers, high-voltage power units are required to switch the polarity of an output voltage in higher speed. To further reduce the polarity switching time in the high-voltage power unit disclosed in Patent Literature 2 stated above, it is necessary, for example, to further increase an overshoot of a negative voltage when a voltage is switched from positive to negative, or to increase the falling speed of a positive voltage (+HV to zero). To achieve the former, it is necessary to increase the voltage generation capability of the negative voltage generating circuit 4. However, this entails such disadvantages as a significant cost hike of the circuits and an increase in necessary electric energy. To achieve the latter, it is necessary to reduce the resistance value of the resistor 51. However, this entails such disadvantages as an increased power loss in the resistor 51 and a necessity of using expensive resistors which can withstand large electric power.
In the high-voltage power unit, an overshot voltage needs to be fully settled (an overshoot needs to be terminated) before the next switching of the polarity is performed. This is because if the next polarity switching is performed before the overshoot is terminated, a fall of a high voltage after an overshoot is added to a next change (rise) in the high voltage in the direction identical to the fall, and thereby it takes a longer time before settling the voltage after all. Therefore, there is a theoretical limit, in the first place, on the attempt of increasing the polarity switching speed by increasing the overshoot.
As a high-voltage power unit of a quite different type, there is also known a unit including semiconductor switches which are bridge-connected as illustrated in FIG. 9. In this high-voltage power unit, switches 21 and 23 are turned on while switches 22 and 24 are turned off for outputting a positive voltage +HV. For outputting a negative voltage −HV, the switches 22 and 24 are turned on while the switches 21 and 23 are turned off To withstand high voltages, the semiconductor switches 21 to 24 are each constructed to include a large number of FETs connected in series. The gate terminal of each of the FETs is driven via a pulse transformer having high insulation, and the like. However, in such a high-voltage power unit, a large number of pulse transformers are needed, and for driving each of these pulse transformers, drive circuits are also needed. Complicated drive control is also needed to prevent a pair of switches 21 and 23 from being turned on at the same time when a pair of switches 22 and 24 is turned on. Therefore, it is unavoidable that such a power unit becomes quite expensive.